In the manufacture of large rotating machines, epoxide resins have long been used as vacuum impregnants for insulation which relies upon mica, in the form of paper, flakes or large splittings as the dielectric. In this art, vinyl modified epoxide-acid anhydride impregnating systems are generally catalyzed with materials such as dicumyl peroxide and tertiary butylperbenzoate, as shown by Mertens, in U.S. Pat. No. 3,647,611, or with quaternary organic phosphonium compounds, as shown by Rogers, in U.S. Pat. No. 3,759,866.
Puchalla, in U.S. Pat. No. 3,244,670, catalyzed epoxides of cyclohexane derivatives with a wide variety of organo-tin halides, used as accelerators, generally in combination with an acid anhydride or phenolic type curing agent. In a somewhat similar fashion, Markovitz, in U.S. Pat. No. 3,622,524, reacted 20 to 80 wt.% of an organo-tin compound, preferably an oxide, with an organic acid or acid anhydride, as a cross-linking agent for epoxide impregnating resins.
Smith et al., in U.S. Pat. No. 4,020,017, eliminated the need for acid anhydride curing or cross-linking agents, in cycloaliphatic or glycidyl ester epoxide systems. Smith used a stablizing, reactive, low viscosity epoxide dilutent, such as neopentyl diglycidyl ether, in conjunction with selected organo-tin halides, to provide low viscosity vacuum impregnating resins for flexible mica insulation used in high voltage stress applications. The Smith resins provided latent catalytic characteristics due to the formation of inner oxonium salts between the organo-tin compound and the diluent.
Vacuum impregnation is costly and time consuming, but has generally been considered necessary to get a void free insulating tape. Groff, however, in U.S. Pat. No. 3,660,220, used a mica-glass cloth tape, impregnated with a solution of an epoxide-caster oil modified acid anhydride, as a flexible pre-preg electrical insulation for motors. The solutions were catalyzed with stannous octoate, tertiary amines or boron trifluoride complexes. These catalysts, however, provide poor electrical dissipation (power) factor values of 25% at 155.degree. C. Also, these mica tapes would not retain their initial flexibility after prolonged storage.
Nishizaki, in U.S. Pat. No. 3,983,289, attempted to solve epoxide pre-preg storage problems. Nishizaki provided arc and track resistant pre-pregs, containing liquid alicyclic epoxy compounds and up to 20 parts per 100 parts epoxy of a wide variety of latent curing agents. The curing agents included salts of Lewis bases, such as triethylammonium acetate; Lewis acids such as boron trifluoride; amines, such as monoethyl amine; phosphines, such as triphenyl phosphine; urea compounds; and long hydrocarbon chain carboxylic acid organo-tin compounds, such as dibutyl tin maleate, dibutyl tin dilaurate and dimethyl tin dioctoate. An epoxy-curing agent solution, after application to a wide variety of solid substrates, such as cloth, paper, asbestos, fiberglass, fibrous sulfates or aluminates, woven plastics and mica paper, among others, was heated between 90.degree. to 140.degree. C to remove solvent. This heating step partly reacts the epoxy and curing agent to form a rigid, B staged resin. This rigid pre-preg is used in board, rod or tube form. They can be used to cover already wound coil insulating tapes or as insertions between foil conductors. They have particular application as arc and track resistant support rods and slot wedges.
While the pre-preg concept is an improvement in the art, it presents a host of problems with respect to tensile strength, void-free resin impregnation, retention of flexibility during solvent removal, and continued flexibility after long periods of storage, i.e. over 6 months. What is needed is a flexible mica-latent catalyzed, resinous insulation winding tape for high voltage motors and large rotating machines, that is void-free, that remains flexible after solvent flash-off, that will retain flexibility after long-term storage, and has sufficient strength and flexibility to be wound onto motor and generator coils.